Magneto-electric machines, including brushless DC motors and conventional DC permanent magnet motors, have been known for many years. A typical magneto-electric machine has phased windings in a stator and permanent magnets on a rotor. The two components are separated by an air gap so that rotation of the rotor may occur.
Brushless and conventional permanent magnet DC motors are described, for example, in U.S. Pat. No. 4,605,874 (Whiteley) as well as in the engineering handbook entitled "DC Motors, Speed Controls, Servo Systems" (fifth edition, 1980) published by Electrocraft Corporation, Hopkins, Minn., U.S.A., and reference may be made thereto to ascertain the current state of the art.
A brushless permanent magnet DC motor requires shaped and time current pulses in its windings in order to provide designed torque characteristics. The average torque is determined in part by the average value of the current pulse, which can be set by amplitude control, by time-ratio-control or a combination of both.
When it is advantageous to employ a half wave winding configuration, each winding carries current for a maximum of one half cycle of alternating counter-emf which arises in the winding due to the relative motion of the magnetic field and winding. Current flow in the other half cycle is not permitted because it contributes negative torque. Thus, prior art control circuits typically utilize a switch in series with each stator coil for alternately pulsing current through the associated coil.
For example, in time-ratio-control systems, energy efficient pulse shaping is effected by the use of fast acting switches in series with the current-carrying windings. However, when the switches are opened, the inductive energy stored in the windings must be decreased or eliminated.
The stored inductive energy may be removed when the switch opens, by absorption in the switch or in an overvoltage device such as a Zener diode, and is typically dissipated as heat.
It is also known from the prior art to utilize a free-wheeling diode connected across the stator coil which becomes forward-biased during the half cycle of the counter-emf when the switch is open, thereby dissipating the stored inductive energy. However, the resulting current flow through the winding produces unacceptable negative torque.